1. Field of the Invention
The invention relates to electrical corona discharge devices and in particular to methods of and devices for fluid acceleration to provide velocity and momentum to a fluid, especially to air, through the use of ions and electrical fields.
2. Description of the Related Art
The prior art as described in a number of patents (see, e.g., U.S. Pat. Nos. 4,210,847 of Spurgin and 4,231,766 of Shannon, et al.) has recognized that the corona discharge device may be used to generate ions and accelerate fluids. Such methods are widely used in electrostatic precipitators and electric wind machines as described in Applied Electrostatic Precipitation published by Chapman and Hall (1997). The corona discharge device may be generated by application of a high voltage to pairs of electrodes, e.g., a corona discharge electrode and an attractor electrode. The electrodes should be configured and arranged to produce a non-uniform electric field generation, the corona electrodes typically having sharp edges or otherwise being small in size.
To start and sustain the corona discharge device, high voltage should be applied between the pair of electrodes, e.g., the corona discharge electrode and a nearby attractor (also termed collector) electrode. At least one electrode, i.e., the corona discharge electrode, should be physically small or include sharp points or edges to provide a suitable electric field gradient in the vicinity of the electrode. There are several known configurations used to apply voltage between the electrodes to efficiently generate the requisite electric field for ion production. U.S. Pat. No. 4,789,801 of Lee and U.S. Pat. Nos. 6,152,146 and 6,176,977 of Taylor, et al., describe applying a pulsed voltage waveform across pairs of the electrodes, the waveform having a duty cycle between 10% and 100%. These patents describe that such voltage generation decreases ozone generation by the resultant corona discharge device in comparison to application of a steady-state, D.C. power. Regardless of actual benefit of such voltage generation for reducing ozone production, air flow generation is substantially decreased by using a duty cycle less than 100%, while the resultant pulsating air flow is considered unpleasant.
U.S. Pat. No. 6,200,539 of Sherman, et al. describes use of a high frequency high voltage power supply to generate an alternating voltage with a frequency of about 20 kHz. Such high frequency high voltage generation requires a bulky, relatively expensive power supply typically incurring high energy losses. U.S. Pat. No. 5,814,135 of Weinberg describes a high voltage power supply that generates very narrow (i.e., steep, short duration) voltage pulses. Such voltage generation can generate only relatively low volume and rate air flow and is not suitable for the acceleration or movement of high air flows.
All of the above technical solutions focus on specific voltage waveform generation. Accordingly, a need exists for a system for and method of optimizing ion induced fluid acceleration taking into consideration all components and acceleration steps.
The prior art fails to recognize or appreciate the fact that the ion generation process is more complicated than merely applying a voltage to two electrodes. Instead, the systems and methods of the prior art are generally incapable of producing substantial airflow and, at the same-time, limiting ozone production.
Corona related processes have three common aspects. A first aspect is the generation of ions in a fluid media. A second aspect is the charging of fluid molecules and foreign particles by the emitted ions. A third aspect is the acceleration of the charged particles toward an opposite (collector) electrode (i.e., along the electric field lines).
Air or other fluid acceleration that is caused by ions, depends both on quantity (i.e., number) of ions and their ability to induce a charge on nearby fluid particles and therefore propel the fluid particles toward an opposing electrode. At the same time, ozone generation is substantially proportional to the power applied to the electrodes. When ions are introduced into the fluid they tend to attach themselves to the particles and to neutrally-charged fluid molecules. Each particle may accept only a limited amount of charge depending on the size of a particular particle. According to the following formula, the maximum amount of charge (so called saturation charge) may be expressed as:
Qp={[(1+2xcex/dp)]2+[1/(1+2xcex/dp)]*[(∈rxe2x88x921)/(∈r+2)]*xcfx80∈0 dp2E,
where dp =particle size, ∈r is the dielectric constant of the dielectric material between electrode pairs and E0 is the dielectric constant in vacuum.
From this equation, it follows that a certain number of ions introduced into the fluid will charge the nearby molecules and ambient particles to some maximum level. This number of ions represents a number of charges flowing from one electrode to another and determines the corona current flowing between the two electrodes.
Once charged, the fluid molecules are attracted to the opposite collector electrode in the direction of the electric field. This directed space over which a force F is exerted, moves molecules having a charge Q which is dependent on the electric field strength E, that is, in turn proportional to the voltage applied to the electrodes:
F=xe2x88x92Q*E.
If a maximum number of ions are introduced into the fluid by the corona current and the resulting charges are accelerated by the applied voltage alone, a substantial airflow is generated while average power consumption is substantially decreased. This may be implemented by controlling how the corona current changes in value from some minimum value to some maximum value while the voltage between the electrodes is substantially constant. In other words, it has been found to be beneficial to minimize a high voltage ripple (or alternating component) of the power voltage applied to the electrodes (as a proportion of the average high voltage applied) while keeping the current ripples substantially high and ideally comparable to the total mean or RMS amplitude of the current. (Unless otherwise noted or implied by usage, as used herein, the term xe2x80x9cripplesxe2x80x9d and phrase xe2x80x9calternating componentxe2x80x9d refer to a time varying component of a signal including all time varying signals waveforms such as sinusoidal, square, sawtooth, irregular, compound, etc., and further including both bi-directional waveforms otherwise known as xe2x80x9calternating currentxe2x80x9d or xe2x80x9ca.c.xe2x80x9d and unidirectional waveforms such as pulsed direct current or xe2x80x9cpulsed d.c.xe2x80x9d. Further, unless otherwise indicated by context, adjectives such as xe2x80x9csmallxe2x80x9d, xe2x80x9clargexe2x80x9d, etc. used in conjunction with such terms including, but not limited to, xe2x80x9cripplexe2x80x9d, xe2x80x9ca.c. component,xe2x80x9d, xe2x80x9calternating componentxe2x80x9d etc., describe the relative or absolute amplitude of a particular parameter such as signal potential (or xe2x80x9cvoltagexe2x80x9d) and signal rate-of-flow (or xe2x80x9ccurrentxe2x80x9d).) Such distinction between the voltage and current waveforms is possible in the corona related technologies and devices because of the reactive (capacitive) component of the corona generation array of corona and attractor electrodes. The capacitive component results in a relatively low amplitude voltage alternating component producing a relatively large corresponding current alternating component. For example, it is possible in corona discharge devices to use a power supply that generates high voltage with small ripples. These ripples should be of comparatively high frequency xe2x80x9cfxe2x80x9d(i.e., greater than 1 kHz). The electrodes (i.e., corona electrode and collector electrode) are designed such that their mutual capacitance C is sufficiently high to present a comparatively small impedance Xc when high frequency voltage is applied, as follows:       X    c    =      1          2      ⁢              xe2x80x83            ⁢      π      ⁢              xe2x80x83            ⁢      fC      
The electrodes represent or may be viewed as a parallel connection of the non-reactive reactive d.c. resistance and reactive a.c. capacitive impedance. Ohmic resistance causes the corona current to flow from one electrode to another. This current amplitude is approximately proportional to the applied voltage amplitude and is substantially constant (d.c.). The capacitive impedance is responsible for the a.c. portion of the current between the electrodes. This portion is proportional to the amplitude of the a.c. component of the applied voltage (the xe2x80x9cripplesxe2x80x9d) and inversely proportional to frequency of the voltage alternating component. Depending on the amplitude of the ripple voltage and its frequency, the amplitude of the a.c. component of the current between the electrodes may be less or greater than the d.c. component of the current.
It has been found that a power supply that is able to generate high voltage with small amplitude ripples (i.e., a filtered d.c. voltage) but provides a current with a relatively large a.c. component (i.e., large amplitude current ripples) across the electrodes provides enhanced ions generation and fluid acceleration while, in case of air, substantially reducing or minimizing ozone production. Thus, the current ripples, expressed as a ratio or fraction defined as the amplitude of an a.c. component of the corona current divided by the amplitude of a d.c. component of the corona current (i.e., Ia.c./Id.c.) should be considerably greater (i.e., at least 2 times) than, and preferably at least 10, 100 and, even more preferably, 1000 times as large as the voltage ripples, the latter similarly defined as the amplitude of the time-varying or a.c. component of the voltage applied to the corona discharge electrode divided by the amplitude of the d.c. component (i.e., Va.c.Vd.c.).
It has been additionally found that optimal corona discharge device performance is achieved when the output voltage has small amplitude voltage alternating component relative to the average voltage amplitude and the current through the electrodes and intervening dielectric (i.e., fluid to be accelerated) is at least 2, and more preferably 10 times, larger (relative to a d.c. current component) than the voltage alternating component (relative to d.c. voltage) i.e., the a.c./d.c. ratio of the current is much greater by a factor of 2, 10 or even more than a.c./d.c. ratio of the applied voltage. That is, it is preferable to generate a voltage across the corona discharge electrodes such that a resultant current satisfies the following relationships:
Va.c. less than  less than Vd.c. and Ia.c.xcx9cId.c.
or
Va.c./Vd.c. less than  less than Ia.c./Id.c.
or
Va.c. less than Vd.c. and Ia.c.  greater than Id.c.
or
VRMSxcx9cVMEAN and IRMS  greater than IMEAN
If any of the above requirements are satisfied, then the resultant corona discharge device consumes less power per cubic foot of fluid moved and produces less ozone (in the case of air) compared to a power supply wherein the a.c./d.c. ratios of current and voltage are approximately equal.
To satisfy these requirements, the power supply and the corona generating device should be appropriately designed and configured. In particular, the power supply should generate a high voltage output with only minimal and, at the same time, relatively high frequency ripples. The corona generating device itself should have a predetermined value of designed, stray or parasitic capacitance that provides a substantial high frequency current flow through the electrodes, i.e., from one electrode to another. Should the power supply generate low frequency ripples, then Xc will be relatively large and the amplitude of the alternating component current will not be comparable to the amplitude of the direct current component of the current. Should the power supply generate very small or no ripple, then alternating current will not be comparable to the direct current. Should the corona generating device (i.e., the electrode array) have a low capacitance (including parasitic and/or stray capacitance between the electrodes), then the alternating current again will not be comparable in amplitude to the direct current. If a large resistance is installed between the power supply and the electrode array (see, for example, U.S. Pat. No. 4,789,801 of Lee, FIGS. 1 and 2), then the amplitude of the a.c. current ripples will be dampened (i.e., decreased) and will not be comparable in amplitude to that of the d.c. (i.e., constant) component of the current. Thus, only if certain conditions are satisfied, such that predetermined voltage and current relationships exist, will the corona generating device optimally function to provide sufficient air flow, enhanced operating efficiency, and desirable ozone levels. The resultant power supply is also less costly.
In particular, a power supply that generates ripples does not require substantial output filtering otherwise provided by a relatively expensive and physically large high voltage capacitor connected at the power supply output. This alone makes the power supply less expensive. In addition, such a power supply has less xe2x80x9cinertiaxe2x80x9d i.e., less stored energy tending to dampen amplitude variations in the output and is therefore capable of rapidly changing output voltage than is a high inertia power supply with no or negligible ripples.